The present invention relates to a control apparatus for a motor.
FIG. 1 is a circuit diagram of an example of a conventional control apparatus for a motor.
Such an apparatus is disclosed in Japanese Patent Application Laid Open No. 56-22254.
The conventional control apparatus comprises stator windings 1a, 1b and 1c of a motor, transistors 2a, 2b and 2c connected in series with the corresponding windings 1a, 1b and 1c, and a control circuit 3 for detecting the number of revolutions of the motor to control the transistors 2a, 2b and 2c in accordance with the detected number of revolutions of the motor. The stator windings 1a, 1b and 1c, the transistors 2a, 2b and 2c, and the control circuit 3 together constitute a motor unit 5.
The control apparatus further comprises a first relay 6, which has changeover contacts 7a, 7b and 7c connected in series with the corresponding stator windings 1a, 1b and 1c, for selectively changing the connection of the windings 1a, 1b and 1c in such a manner that electric power having a voltage +B is supplied to the stator windings 1a, 1b, and 1c and that the motor unit 5 is operated in a dynamic braking mode, a switch 8 for actuating the relay 6, a second relay 9, having changeover contacts 10a, 10b and 10c, connected in series with the corresponding stator windings 1a, 1b and 1c of the motor unit 5, control resistors 11a, 11b and 11c being connectable in series with the stator windings 1a, 1b and 1c when the motor unit 5 is operated in the dynamic braking mode, a power rectifying circuit 12, which is composed of capacitors 13a, 13b, diodes 14a, 14b, 14c, a variable resistor 15a and a resistor 15b for supplying a regenerative electric power to the relay 9 when the motor unit is operated in the dynamic braking mode.
When the switch 8 is turned ON, the first relay 6 is energized so as to connect the contacts 7a, 7b and 7c to the contacts 16a, 16b and 16c, whereby electric power having a voltage +B is applied to each of the stator windings 1a, 1b and 1c, whereby each phase of the respective windings is controlled so that the motor begins rotating. During this time, the power rectifying circuit 12 supplies a power to the relay 9, and the variable resistor 15a is adjusted such that the rectifying circuit 12 supplies a sufficiently large current to energize the second relay 9.
When the switch 8 is turned OFF, the first relay 6 is de-energized so as to connect the contacts 7a 7b and 7c of the relay 6 to the contacts 8a, 8b and 8c and the motor unit 5 is used as a dynamic generator. During this time, the second relay 9 is energized so as to connect the contacts 10a, 10b and 10c to the resistors 11a, 11b and 11c, thereby establishing a short-circuit in the respective stator windings 1a, 1b and 1c. When the voltage across the terminals of the second relay 9 becomes below a predetermined value, the second relay 9 is de-energized so that the contacts 10a, 10b and 10c are contacted to the contacts 17a, 17b and 17c, thereby establishing a complete short-circuit in the respective stator windings 1a, 1b and 1c, whereby the motor unit 5 is quickly stopped.
The thus constructed conventional control apparatus for a motor has problems in that the control apparatus needs changeover contacts equal in number to the number of the stator windings or more, thereby causing the apparatus to be complicated, encumbrant and expensive.